The present invention is directed to a process and catalyst for shape selective hydrocarbon conversions such as the regioselective conversion of toluene to para-xylene.
The term shape-selective catalysis describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g. by N.Y. Chen, W. E. Garwood and F. G. Dwyer, "Shape Selective Catalysis in Industrial Applications," 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization and aromatic disproportionation, alkylation or transalkylation reactions are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. Another type of selectivity results from configurational diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in toluene selective disproportionation to p-xylene.
Para-xylene is a very valuable commercial product useful in the production of polyester fibers. The catalytic production of para-xylene has received much attention in the scientific community and various methods for increasing catalyst para-selectivity have been described.
The synthesis of para-xylene is typically performed by methylation of toluene over a catalyst under conversion conditions. Examples are the reaction of toluene with methanol as described by Chen et al., J. Amer. Chem. Sec. 1979, 101, 6783, and toluene disproportionation, as described by Pines in "The Chemistry of Catalytic Hydrocarbon Conversions", Academic Press, N.Y., 1981, p. 72. Such methods typically result in the production of a mixture including para-xylene, ortho-xylene, and meta-xylene. Depending upon the para-selectivity of the catalyst and the reaction conditions, different percentages of para-xylene are obtained. The yield, i.e., the amount of feedstock actually converted to xylene, is also affected by the catalyst and the reaction conditions.
The equilibrium reaction for the conversion of toluene to xylene and benzene proceeds as follows: ##STR1##
One known method for increasing para-selectivity of zeolite catalysts is to modify the catalyst by treatment with "selectivating agents". Modification methods have been suggested wherein the catalyst is modified by treatment prior to use to provide a silica coating. For example, U.S. Pat. Nos. 4,477,583 and 4,127,616 disclose methods wherein a catalyst is contacted at ambient conditions with a modifying compound such as phenylmethyl silicone in a hydrocarbon solvent or an aqueous emulsion, followed by calcination. Such modification procedures have been successful in obtaining para-selectivity of greater than about 90% but with commercially unacceptable toluene conversions of only about 10%, resulting in a yield of not greater than about 9%, i.e. 10% .times.90%. Such processes also produce significant quantities of ortho-xylene and meta-xylene thereby necessitating expensive separation processes in order to separate the para-xylene from the other isomers.
Typical separation procedures include costly fractional crystallization and adsorptive separation of para-xylene from other xylene isomers which are customarily recycled. Xylene isomerization units are then required for additional conversion of the recycled xylene isomers into an equilibrium xylene mixture comprising para-xylene.
Those skilled in the art appreciate that the expense of the separation process is proportional to the degree of separation required. Therefore, significant cost savings are achieved by increasing selectivity to the para-isomer while maintaining commercially acceptable conversion levels.
It is, therefore, highly desirable to provide a regioselective process for the production of para-xylene from toluene while maintaining commercially acceptable toluene conversion levels.